This invention relates to gas cooled nuclear reactors. Gas cooled nuclear reactors such as the AGR employing metal canned fuel have now been developed to yield an average outlet gas temperature of 650.degree. C., high enough for raising steam of a quality suited for modern turbines. The High Temperature Gas-cooled Reactor (HTR) which is still under development uses graphite clad fuel elements containing fission product-retaining particles or so called coated particles and offers the prospect of yielding an outlet gas at a temperature of about 750.degree. C. This increase in gas temperature is substantial but nevertheless the higher temperature is not essential for contemporary steam turbines and is not high enough for economical use in the more important process heat applications, such as iron smelting nor for use as working fluid in a gas turbine cycle. In both instances quoted a gas temperature of about 900.degree. C. or above is desirable and it would seem that until a significant improvement is made the exploitation of HTR technology in these fields will be difficult.
However, the coated fuel particle which forms the basis of HTR fuel is an empirically developed item whose correct performance at high temperature is vital to sustain the heat generating process in the reactor core and the technical limits for its reliable operation have probably been reached. In fact, it is doubtful, even if a further lengthy period of development were carried out, whether the necessary increase in fuel operational temperature could be achieved. If this be so then there exists a requirement for some means of enabling the reactor outlet gas temperature of the HTR to be raised whilst keeping the particle temperature substantially unchanged thereby maintaining the integrity of the particle. Once this has been accomplished, current HTR technology will be more readily exploitable.
However, nuclear fuel designers operate under certain constraints which tend to conflict. Uniformity of performance and especially the achievement of the lowest practicable temperature drop between fuel and coolant, .DELTA.T, requires high precision in manufacture. But the cost of HTR fuel tends to be high compared with metal clad fuel and a main source of this lies in the complex machining and inspection required to achieve precision with the fuel element configurations proposed hitherto. A desirable manufacturing route would be one which yields a precisely dimensional product using simple automated processes both for fabrication and inspection. Such a route would assist in reducing both .DELTA.T and the fabrication cost.
Several methods of manufacturing HTR fuel have been proposed hitherto based on cylindrical fuel compact designs. These designs and their methods of manufacture result in a fuel compact which presents a relatively large temperature difference (.DELTA.T) between fuel particles and the cooled surface of the cladding. This is due in the case of the "tubular interacting" type of fuel element to a relatively thick graphite layer between fuel and coolant and to the unavoidable clearance gaps between the fuel compact and its enclosing graphite cladding. In the "integral block design" of fuel bearing member the thermal resistance of the graphite block itself acts as an appreciable barrier to heat transfer between fuel compacts contained in holes in the block and those holes through which the coolant passes. One prior proposal which eliminates the contribution to the .DELTA.T made by the clearance gap between a cylindrical fuel compact and its cladding, resides in adopting an isostatic pressing together of these components in the green state. However, this technique results in a high degree of non uniformity both on the exterior of the pressed product and on the internal boundaries between fuelled and fuel free layers. The former is remedied by surface machining which is intrinsically costly while the latter leads to high mean thicknesses for the fuel free layers giving high .DELTA.T.
The characteristics of various HTR fuel designs have been discussed in the Proceedings of NUCLEX, an exhibition held at Basle, Switzerland, 16-21 Oct. 1972; see in particular Paper 3/13, "HTR Fuels and their future development" by L W Graham and M S T Price.